I don't see any sane reason for some of those to be PGDLLIMPORT but not others. In particular it's pretty silly for IsBackgroundWorker not to be PGDLLIMPORT.
Too trivial to be worth making an actual patch, it'd be more work to apply it than edit directly.
My guess is that PGDLLIMPORT has been added explicitly when somebody needed it for something, without any actual thought. I can't say I see any reason not to export the other ones as well -- more that maybe there are even more that are needed?